Single Avalanches Research Articles

On the deterministic and stochastic solutions of Space Charge models and their impact on high resolution timing

RPCs offer unique opportunities to investigate basic processes in gaseous electronics. The growth of a single avalanche can be studied in a regime where it reacts to its own field. This induces a saturation in its development, often described in a deterministic scenario by a nonlinear model. Once reinterpreted in a fully stochastic framework, the same feature corresponds to a negative feedback mechanism, which regulates the avalanche development and preserves its timing properties. Fluctuations are hence mostly produced in the initial phase of the growth. A clear evidence of the action of this stabilizing scheme is observed in data collected for single avalanches of fixed length.

Detailed RPC avalanche simulations

We report on the progress in the simulation of Resistive Plate Chambers (RPCs). We investigate the influence of the space charge effect on the signal rise and the time resolution. A two-dimensional simulation model allows to study in detail the development of single avalanches.

Effects of photoionization on propagation and branching of positive and negative streamers in sprites

Modeling studies indicate that double‐headed streamers originating from single electron avalanches in lightning‐driven quasi‐static electric fields at mesospheric altitudes accelerate and expand, reaching transverse scales from tens to a few hundreds of meters and propagation speeds up to one tenth of the speed of light, in good agreement with recent telescopic, high‐speed video and multichannel photometric observations of sprites. The preionization of the medium ahead of a streamer by the ionizing UV photons originating from a region of high electric field in the streamer head (i.e., photoionization) significantly modifies the streamer scaling properties as a function of air pressure in comparison with those predicted by similarity laws. The photoionization leads to lower peak electric fields in the streamer head, lower streamer electron densities, wider initial streamer structures, and lower acceleration and expansion rates of streamers at sprite altitudes 40–90 km, when compared to the ground level. The primary reason for the observed differences is that the effective quenching altitude of the excited states of the molecular nitrogen b1Πu, b′1, and c′41 that give photoionizing radiation is about 24 km. The quenching of these states is therefore negligible at sprite altitudes, leading to a substantial enhancement of the electron‐ion pair production ahead of the streamer tip because of the photoionization, when compared to the ground level. The maximum radius of the expanding streamers is predominantly controlled by the combination of the absorption cross section χmin = 3.5 × 10−2 cm−1 Torr−1 of the molecular oxygen (O2) at 1025 Å and the partial pressure of O2 in air, Streamers exhibit branching when their radius becomes greater than Model results indicate a lower branching threshold radius for positive streamers in comparison with negative streamers, under otherwise identical ambient conditions. These results are in good agreement with recent results of high‐speed photography of laboratory streamers in near‐atmospheric pressure N2/O2 mixtures and similar morphology documented during recent telescopic and high‐speed video observations of sprites.

Two types of avalanche behaviour in granular media

The nature of the transition between static and flowing regimes in granular media1,2 provides a key to understanding their dynamics. When a pile of sand starts flowing, avalanches occur on its inclined free surface. Previously, studies3 of avalanches in granular media have considered the time series of avalanches in rotating drums4, or in piles continuously fed with material. Here we investigate single avalanches created by perturbing a static layer of glass beads on a rough inclined plane. We observe two distinct types of avalanche, with evidence for different underlying physical mechanisms. Perturbing a thin layer results in an avalanche propagating downhill and also laterally owing to collisions between neighbouring grains, causing triangular tracks; perturbing a thick layer results in an avalanche front that also propagates upwards, grains located uphill progressively tumbling down because of loss of support. The perturbation threshold for triggering an avalanche is found to decrease to zero at a critical slope. Our results may improve understanding of naturally occurring avalanches on snow slopes5 where triangular tracks are also observed.

The barrier effect on the onset voltage of negative corona in air

This paper is aimed at calculating the onset voltage of negative corona in rod - plane gaps as influenced by the presence of a dielectric barrier located either parallel or normal to the gap axis. The effect of the barrier thickness, permittivity and location with respect to the stressed rod on the calculated onset voltage is investigated. The method of calculation is based on the criterion of self-recurring single electron avalanches developed in the gap when stressed negatively. The electric field values used for avalanche growth are evaluated using a combined technique based on boundary-element and discrete-charge simulations. In this technique, the surface charge on the rod is simulated by fictitious discrete charges while the surface charge on the faces of the barrier is simulated by surface charged boundary elements. The electric field assumes values higher than those in the absence of the barrier whatever the position of the barrier, parallel or normal to the gap axis. The smaller the distance between the barrier and the rod, the larger the thickness and the higher the relative permittivity of the barrier, the lower is the corona onset voltage.

Calculation of corona onset voltage for duct-type precipitators

A pure theoretical method for calculating the onset voltage of corona in duct-type electrostatic precipitators that is independent of the arrangement of discharge wires relative to the collecting plates is described. This method is based on a criterion for self-recurring single electron avalanches in a known electric field distribution in the ionization zone surrounding the discharge wire. The results computed by this method are in reasonable agreement with those measured experimentally. >

Avalanches, hydrodynamics, and discharge events in models of sandpiles.

Motivated by recent studies of Bak, Tang, and Wiesenfeld [Phys. Rev. Lett. 59, 381 (1987); Phys. Rev. A 38, 364 (1988)], we study self-organized criticality in models of ``running'' sandpiles. Our analysis reveals rich temporal structures in the flow of sand: at very short time scales, the flow is dominated by single avalanches. These avalanches overlap at intermediate time scales; their interactions lead to 1/f noise in the flow. We show that scaling in this region is a consequence of conservation laws and is exhibited in many examples of driven-diffusion equations for transport. At very long time scales, the sandpiles exhibit system-wide discharge events. These events also obey scaling and are found to be anticorrelated. We derive the ${\mathit{f}}^{1/2}$ mean-field power spectrum for these events and show that a threshold instability of the model, coupled with some stochasticity, is the underlying origin of the long-time anticorrelation.

A highly efficient low-pressure UV-rich detector with optical avalanche recording

UV photons from a Cherenkov radiator are multiplied in a multistep avalanche chamber operating in a gated mode at low gas pressure (40 Torr). The gas mixture is C 2H 6-argon ( 80 20 ) and TMAE at 34°C. Visible light emitted from single photoelectron avalanches is detected by a CCD camera coupled to an image intensifier system. The detector was tested with 5 GeV c electrons, using a CH 4 radiator gas at 1 atm. Cherenkov rings essentially free of particle background and of secondary photon feedback were obtained in this mode of operation with a mean number n ≅ 11.5 ( N 0 ≅ 76 cm −1). We present this new method and discuss its performance.

Probability density distribution of primary ionization calculated by means of an iterative deconvolution process

Abstract The probability density distribution of primary ionization in a proportional counter has been obtained by an iterative deconvolution process of the measured pulse height distribution using the single electron distribution. The Polya distribution function gave the best fit to measured single electron spectra and was used to describe the shape of single avalanches in gases. Results of distributions of primary ionization in methane and propane are presented.

Electrostatic imaging of minimum ionizing particles

Abstract Charge amplification in a multistep chamber permits the electrostatic imaging of minimum ionizing particles trajectories in various gases at atmospheric pressure. In helium the single avalanches from primary electrons are easily detected.

Charges on insulators generated by breakdown of gas

The conditions for the occurrence of partial discharges are analysed and shown to be governed by the trapping of charged particles on the insulation surface. According to the instant of trapping, each discharge is constituted either by a series of single avalanches or by a sort of degenerated spark which constitutes the partial discharge.Charge deposits form a sharply defined site which is visible on powdering. If the deposited charge is positive, the site is star-shaped and its shape and size strongly depend on the electronegativity of the gas. If the discharge is negative, the site is circular in all the gases. By measuring the site diameter it is possible in several cases to calculate the capacity involved in the discharge, and consequently the energy of it. This technique is used to study the explosivity of vapours in spaces bounded by charged insulators.

The statistical distribution of particle number in an avalanche chain

An expression has been derived for the probability distribution of the total number of electrons and ions in the avalanche chain which develops in a low pressure gas in a high electric field when single avalanches generate successors by photo-electric effect at the cathode. The analysis is based on the assumption that individual avalanche distributions are governed by a simple type of compound Poisson law. The resultant formula has been shown to give correct results when applied to the two special cases for which independent calculations are available.

Untersuchung der Elektronenkomponente von Einzellawinen mit dem Photomultiplier

With a photomultiplier single electron avalanches of 103 carriers can be observed in nitrogen. 2.5% of all photons (in the range 3000≦λ≦7000 A) of an avalanche produce a photoelectron at the cathode of the photomultiplier. The influence of the variable solid angle on the photomultiplier pulse is given. Trouble by noise is calculated and it is shown that the information depends on the number of photoelectrons only. The electron components of avalanches in methane, nitrogen, and mixtures have an exponential rise and the measured rise time constant τbeob is in accordance with the theoretical value 1/αυ_. For carrier numbers >106 space charge influence is observed, as given by theory. The quantum efficiencyQ per ionizing impact is found to beQ∼10−3 in vapours andQ∼1 in gases. Values of the electron drift velocity in mixtures of N2 and CH4 are given.

Experimental Townsend Ionization Coefficients for Isopentane

Townsend α and γ values have been measured over the range 60≤E/p0≤3000 for isopentane using the dc method of measuring the ionization current. The experimental apparatus and procedure are described. It is shown that Townsend's law α/p0=A exp (—Bp0/E) is obeyed within 80≤E/p0≤500. Positive ion action at the cathode sets in at E/p0≥500. The results agree with previous breakdown measurements. It was found possible to follow the current growth due to ionization of the gas over a multiplication range exceeding eight orders of magnitude. The influence of space-charge distortion within single avalanches and that due to neighboring ones has been examined. No space-charge reduction of the avalanche multiplication was observed until eαd≥107.

Die Statistik der Elektronenlawinen in elektronegativen Gasen, bei hohen Feldstärken und bei großer Gasverstärkung

The statistical distribution of the carrier number of single electron avalanches in a TOWNSEND discharge is described by v(n) = 1/n·exp (—n/n̄) if one introduces some simplifying assumptions. These assumptions are violated in the case of electronegative gases, in strong electric fields, and in the case of large gas-amplification. In electronegative gases only a part of the primary electrons form observable electron avalanches. These are still subject to an exponential distribution but with an increased mean value. In strong electric fields the ionization probability depends on the previous history of the individual electrons. This leads to a distribution with a marked maximum and a reduced dispersion. In a first approximation the form of the distribution is determined by the quotient E/α: Ui. In the case of large gas-amplification the further development of the avalanche is influenced by the space charge and one gets a modified exponential distribution. The calculated distributions agree well with the experiments of other authors.

�ber den zeitlichen Verlauf der Lichtemission einer Gasentladung

By means of a photomultiplier the temporal course of the light emission due to electron impact is observed in vapours as methane and methylal. It is thus possible to observe single avalanches, their successors and the transition to breakdown. In methylal the rise time constant of light emission of single avalanches is compared with the rise time constant of the electron production measured by the electrical method. These rise time constants are found to be equal. By this way a quantum efficiency of 3.5 · 10−4 photons per avalanche electron could be derived in methylal. The growing rate of single avalanches of 108 electrons is shown to be decreased by space charge. —In methane series of avalanches were investigated: The electron drift velocity (v_=bp · E/p;bp=2.3 · 105 Torr cm2/Vsec,E/p:40 bis 110 V/Torrcm) and the average life time of excited states (∼7 · 10−9 sec) were measured. In methane the breakdown is produced by series of avalanches, whereas in methylal with its smallerγ the rapid increase of light emission without successors indicates a breakdown mechanism by streamer formation.